Heat treating magnesium alloy



Patented July 13; 1926.

1 UNITED STATES PATENT oFFic a. 1

i, ZAY JEFFRIES AND ROBERT S. ARCHER, OF CLEVELAND, OHIO, ASSIGNORS TOAMERI- CAN MAGNESIUM CORPORATION, OF NIAGARA FALLS, NEW YORK, ACORPORATION OI NEW YORK.

Io Drawing.

Our invention relates to the heat treatment of magnesium base alloys,that is, those in which the principal metal is magnesium, a m ng its.objects the improvement 5 of certain of the'physical properties of suchalloys and particularly the hardness and strength.

In a copending application,.S. N.'507,7 66,

filed October 14, 1921, we have described the heat treatment ofmagnesium base alloys, to improve certain of their physical properties,such as the plasticity or strength.

Our present discovery-is preferably used in connection with thatprocess, although it is .not so limited, but is otherwise susceptible ofgeneral application; Generally speaking, the process comprisessubjecting magnesium base alloys to heat treatmentat a temperaturereferably of appro ximately 150 degrees for an extended period of'time.Considerable variation is permissible inthe temperature for securing thedesired changes in the characteristics of the treated metal andaccordingly it can beas I low as 125 degrees C. and as high as 300degrees C. If the temperature of treatment is too low, changes in thecharacteristics of the metal will not occur or will require a period oftime too long perature of treatment is too high, there is danger of notobtaining the desired hardness; either that or temporarily obtaining it,but passing later into a softer conditlon. The process is not limited toany definite magnesmm base alloy, although certain alloys are found tobe particularly susceptible to improvement bytreatment of the characterdescribed. An investigation of the alloys of ma esium' with aluminum hasshown that w en-the content of aluminum is 6% or'less, the hardeningwhich is obtained is insignificant. With an aluminum content of 8% ormore, improvement results from the treatment described, the effectincreasing with the contentof aluminum at least up to about 12%aluminum. With alloys; containing 12% aluminum, the amount and rate ofhardening are found to be the greatest. Alloys containing 4% aluminumand 4% zinc have also been found to be hardened by this heat treatment.

On the other hand, the presence of calcium 0 in such alloys beyond acertain amount infor commercial practice. On the other hand, if thetemnear TREATING MAGNESIUM ALLOY.

hibits hardening. It is sometimes desirable to use calcium in connectionwith casting of the metal to prevent burning of the magnesium duringboth melting and casting of the metal and also during higher temperatureheat treatments. In treating castings according to our process, we findthat the chief effect of calcium up to 0.5% is to decrease the rate ofhardening rather than the ulti mate hardness.

In the practice of our process, it is desirable that the alloy be inproper receptive condition before the application of the low temperatureheat treatment, and while prior heat treatment at a temperature of400-440 degrees C. as described in our copen'ding application, ispreferable it is not entirely necessary that these precise heattreatments be appliedas it is possible that the process of manufacturemay be such that the alloy is left in a condition susceptible to ourprocess without the necessity of the previous heat treatment. U

;As we have described our process, the treatment is applied as adistinct operation and is essentially a heating operation applied afterthe alloy has cooled from the temperature of fabrication or otherprocesses involving the application-of heat.

The same improvement in the properties i of the metal may be obtained,however, by so regulating the rate of cooling from a period of time.'

The'rate or character of cooling of the alloy from the temperature atwhich the hardening treatment is carried out is of practically noimportance from the standpoint of hardness or increased strength.

Some of the difi'erent applications of-our process may be illustratedby, the following examples: p

Emample 1.-A piston was cast in an iron mold from an alloy containing12% aluminum, the balance being nearly pure magnesium. After the castinghad cooled to the ordinary temperature, it was found 'to have a Brinellhardness of 71. It was then sub ected to a heat treatment process asdescribed in our copending application, which consisted inheating for aperiod of 15 hours at 420 degrees C. and quenching in water; The Brinellhardness t on was then found to be 111.

' lowin in water, the Brinell 'a tensile strength 'of 56,600

Example 2. An alloy of magnesium with 10% aluminum was extruded into rodinch in diameter. This rod was found to possess the following physicalcharacteristics: tensile strength 47,600 pounds per square inch,elongation 7.3% and =Brinell hardness'70. Some of this rod was heatedfor hour at 420 degrees C. ,and quenched in water. The physicalcharacteristics were then: tensile strength 49,600 pounds per squareinch, elongation 10.6% and Brinell hardness 58. Some of this heattreated rod was then subjected to a hardeningtreatment at 150 degreesfor 15 hours. The hardness and strength were then found to have beenmarkedly increased, as shown by the folvalues: tensile strength, 60,450poun s per square inch, elongation 2.7% and Brinell hardness 91.

Example 3.An alloy containing 4% aluminum and 4% zince was extruded intorod inch in diameter. Its Brinell hardness was found to be 60. Afterheating for hour at 420 degrees C. and quenching hardness was 54. Someof this heat treated rod was then re heated for 15 hours at 150 degreesC. after gvh izch its Brinell hardness was found to Example 4.A 10%aluminum alloy in the extruded condition had a" tensile strength of47,600 pounds per square inch, with an elongation of 8.3% and a Brinellhardness .of 71. Some of the rod was subjected to ahardening treatment,consisting 1n heating for 15 hours at 150 degrees C. after WlllCll itwas found that the Brinell from 80 to 94. One particular test bar poundsper square inch, with an elongation of 2.7% and a Brinell hardness of74. F

Example 5.--The Brinell hardness of the hardness was increased to valuesranging piston as cast was 71. After the application of our process asdescribed in Example 1, the hardness was increased to 111. The pistonwas then reheated for a period of 2 hours at 420 degrees C. If quenchedfrom this temperature,-the hardness would have been approximately 66.Instead of quenching, th iston was allowed to cool inair. During t iscooling it was, of course, subjected to a certain sojourn in atemperature range suitable for hardening. After cooling to roomtemperature, ness was found to be 80.

EwampletZ-A. piece of 12% aluminum the case of quenched specimens.

the Brinell hardslower than in the case of specimens quenched in waterfIOml420 degrees C. and the maximum hardness reached was lower than inTheory of operation. Aluminum is soluble in solid magne- 'sium up toabout 11 to 12% at the eutectic temperature of 435 degrees C. Thesolubility'decreases as the temperature falls, the excess aluminum beingprecipitated in the form of a hard constituent. A magnesium alloycontaining the aforesaid percentage of aluminum does notcontain all ofthis aluminum in solution after normal solidification from the moltenstate. Alloys containing about 6% or more of aluminum when examined inthe as cast condition are found to contain particles of the hardconstituent, the amountincreasing with the aluminum content. The processof solidification is such that the first material to freeze containsvery little aluminum and as the freezing progresses the aluminum contentof the metal freezing out increases. The resulting structure is anaggregate of .grains of magnesium containing some aluminum in solutionand partially surrounded by a net work of the hard and brittleconstituent which envelope the grains of the first mentioned material,the extent of the envelopment being somewhat in proportion to thealuminum content. Since the network is hard and strong, it impart-sstiffness and hardness .to the alloys'and when not in too great quantityis a source of strength. Large amounts of the brittle network, however,causes brittleness of the alloys as a whole, with consequent diminishedstrength. It is, therefore, found that the maximum tensile strength ofcast alloys occurs at about 7% aluminum although the hardness continuesto increase up to at least 16% and probably beyond.

Although the network is completely solulble at the eutectic temperature,it is not possible to obtain complete solution merely 11y rocess isnecessary w ich re uires heating the alloy -at this tem erature.diffusion considera le time. It is impracticab e to heat treat at thistemperature on account of the danger of melting the eutectic portions ofthe alloy before solution and diffusion are complete. plication, wehave'found that a suitable temperature for alloys containing up to 12%aluminum is from 400- degrees to 440 degrees C. Prolonged heating atthis temperature As set forth in our copending ap-- results in thesolution of some of the brittle network anddifiusion through the mass ofthe alloy, so that eventually a homogeneous solid solution 1s obtained.The solution-and diffusion of the hard network is found to improve thetensile strength and plasticity of the alloys. The Brinell hardness isin most cases slightly reduced. The elastic limit and yield point mayalso be reduced.

- This heat treatment will hereinafter be refied heat treatment which isunstable at lowferred to as the solution heat treatment. The solidsolution obtained by the specier temperature can be preserved by rapidcooling from the heat treating temperature. After quenching, changes inthe characteristics of the metal, however, do not occur spontaneously,as in the case of some of the light aluminum alloys, but'require theapplication of heat, the necessary temperature for the specified alloysbeing from 1,25-

' 300 degrees C.

In heat treating a solid solution at temperatures of 125 to 300 degreesC. the increase in hardness is understood to be attributable to theprecipitation of the hard constituent in the form of very fineparticles. The available evidence indicates that this solid solution ismade up structurally of crystalline magnesium in which some of the atomsare replaced by atoms of aluminum,

the aluminum being in a state of atomic dispersion. When precipitationoccurs, some of the aluminum. atoms and magnesium atoms becomeassociated in a new space lattice, forming a new crystalline constituentof different physical properties, namely of much greater hardness andless plasticity than the magnesium rich solid solution. The mmutecrystals of the new phase grow in accordance with the usual laws ofcrystal "'rowthon prolonged heating atsufiiciently high temperatures.his heat treatment will be referred to as the precipitation heattreatment.

All evidence indicates that the greatest hardness is produced by thefinest crystalline particles and that the hardness decreases as theseparticles grow. It is therefore desirable to produce precipitation ascompletely as possible, but to avoid further growth of the particles asmuch as possible. This result is best at a low temperature such as 150degrees C.

rather" than by a briefexposure at a high temperature such' as 300degrees C. YVhile the maximum results from the precipltatlon heattreatment are obtainable after the solution heat treatment, yetappreciable and important results from the precipltatlon heat treatment,can result of the nature of the cooling process in casting or otherwisefabricating the alloy, if a suflicient amount of aluminum has beenretained in the solid solution during cooling.

The change which takes place during the precipitation treatment is an,increase in hardness, elastic limit. and tensile 'strength accomplishedby a, long exposure tainin be had, as a with a decrease in plasticity.When the process is carried to completion the ductility is decreased toa marked extent, although this may not be true atall stages of theprocess. The rocess istherefore applicable in any case where it isdesired to in-- base alloys comprising subjecting the metal to heattreatment at a temperature of 125 to 300 degrees C. to causeprecipitation of a hard constituent in the form of very small particles.

3. The process of heat treating magnesium base alloys to increase thehardness by subjecting the metal to a temperature of approximately 150degrees C. for 15 to 40 ours.

4. The'process of changing certain of the ultimate physical propertiesof magnesium base alloys comprising subjecting the. metal to heattreatment at a temperature of 125 to 300 degrees C. for approximately 15to 40' hours.

5. The process of. increasing the hardness and strength of magnesiumbase alloys containing from 3 to 15% aluminum comprising subjecting themetal to heat treatment at a temperature of approximately 150 degrees C.

6. The process of'increasing the hardness and strength of magnesium basealloys con taining from 3 to 15% aluminum comprising subjecting themetal to heat treatment at a temperature of approximately 125 to 300degrees C.

7. The prbcess of increasing the hardness and strength of magnesium basealloys con from 3 to 15% aluminum compr1sing su jecting the metal toheat treatment at a temperature of approximately 150 de grees' C. for 15to 40 hours.

8. The process of increasing the hardness and strength of magnesium base,alloys containing from 3 to 15% aluminum comprising subjecting themetal to heat treatment at a temperature of approximately 125 to Y 300degrees C. up to 40 hours.

9. The process of changing certain of the physical properties ofmagnesium base aloys by subjecting the metal to heat treatment at atemperature of approximately 150 degrees C. for a period of timesuflicient to cause substantial improvement in its hardness.

10. The process of changing certain of the physical properties ofmagnesium base alloys by subjecting the metal to heat treatment at atemperature of approximately 125 to 300 degrees for a period of timesuflioient to cause substantial improvement in its hardness.

to a temperature of approximate y 150 degrees C. to increase itshardness.

12. The process of changing certain of the.

physical characteristics 0 magnesuim base alloys containing aluminum,comprising subjecting the metal to heat treatment at a temperature ofapproximatel 125 to 300 C. for a 'eriod of time su cient to causeprecipitation of a hard constituent.

13, The rocess of changing certain of the physicaq characteristics ofmagnesium base alloys containing aluminum, comprising subjecting themetal to heat treatment at a temperature of approximately 400 to 440 C.to secure a solid solution of magnesium and aluminum which will besupersaturated. at room temperatureand thereafter to heat treatment at atemperature of approximately 125 to 300 C. for a period of timesufficient to cause precipitation of a hard constituent.

14. An alloy containing magnesium as the principal metal andcharacterized by the presence of a hard magnesium containing 40constituent in the form of very small particles. I p

15. An alloy containing magnesiumas the principal metal and containingaluminum from three to fifteen percent and characterized by the presenceof a hard magnesiumaluminum constituent in the form of very smallparticles.

16. The process of treating magnesium base alloys to improve certain ofthe physical properties comprising subjecting the metal to heattreatment above 400 degrees C. and thereafter subjecting it for a periodof time at a lower temperature to increase its hardness.

17. The process of improving certain of the physical properties ofmagnesium base alloys and particularly the hardness, com-.

prising the heat treatment of a supersaturated solid solution of themetals at a temerature ofapproximately 125. to 300 C.

or a cipitati'on of a hard constituent therefrom in the form of verysmall particles.

18. The process of improving certain of the physical properties ofmagnesium base alloys and particularly the hardness, comprising the heattreatment of a supersaturated solid solution of magnesium and alu--minum at a temperature of approximately 125 to 300 C. for a period oftime suflicient to cause precipitation of a hard constituent therefromin the form of very small particles.

In testimony whereof we aflix our signatures.

ZAY JEFFRIES. I ROBERT S. ARCHER.

period of time suflicient to cause pre--

